Activity coefficient of carbon in Fe-Co-Ni-C alloys at 1273 K
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E x p e r i m e n t a l data have b e e n obtained for the c o n c e n t r a t i o n of c a r b o n over the e n t i r e r a n g e of F e - C o - N i a l l o y s at 1000~ and a c a r b o n a c t i v i t y of 0.71. A c o m p a r i s o n of the c a p a b i l i t y of a C e n t r a l A t o m s Model and a s i m p l e r i n t e r p o l a t i o n s c h e m e to p r e d i c t the r e s u l t s is m a d e . The c o n v e n i e n c e of the l a t t e r method with v i r t u a l l y no l o s s of a c c u r a c y m a k e s it m o r e s u i t a b l e for p r e d i c t i o n p u r p o s e s .
E S T I M A T I O N of the t h e r m o d y n a m i c p r o p e r t i e s of m u R i c o m p o n e n t a l l o y s f r o m data o b t a i n e d on b i n a r y and t e r n a r y a l l o y s has r e c e i v e d c o n s i d e r a b l e a t t e n tion in r e c e n t y e a r s . R e v i e w s have b e e n p r e s e n t e d by A n s a r a 1 and S p e n c e r et al. 2 The e q u a t i o n s p r o p o s e d for data e x t r a p o l a t i o n have b e e n u s u a l l y m o r e a p p r o p r i a t e for s u b s t i t u t i o n a l e l e m e n t s than for i n t e r s t i t i a l s o l u t e s . A b r i e f s u m m a r y of work r e l a t i n g to the l a t t e r c l a s s of a l l o y s is g i v e n below. D a r k e n 3 and A l c o c k and R i c h a r d s o n 4 used the r e g u l a r s o l u t i o n model for s u b s t i t u t i o n a l s o l u t i o n s in d e r i v i n g Eq. [1] but applied it to i n t e r s t i t i a l and e l e c t r o negative s o l u t e s : loge721-2-3 : Xllogey21-2 + XalOgeu z-3 -- ,sGE_a/RT
[1]
where X1, X3 r e f e r to mole f r a c t i o n s of c o m p o n e n t s 1 E and 3, LxGI_3is the e x c e s s f r e e e n e r g y of mixing in the b i n a r y 1-3 and 721-2-3 is the a c t i v i t y coefficient of solute 2 in the s y s t e m 1 - 2 - 3 . F o r the c a s e , in which the i n t e r s t i t i a l solute o c c u pying a s e p a r a t e s u b l a t t i c e is s p e c i f i c a l l y t a k e n into account in the z e r o t h a p p r o x i m a t i o n , 5 the AGEs t e r m d r o p s out f r o m Eq. [1]. The o m i s s i o n of AGE_3 a r i s e s f r o m no change in the n u m b e r of 1-3 c o n t a c t s when an i n t e r s t i t i a l solute is i n t r o d u c e d , c o n t r a r y to the c a s e of a s u b s t i t u t i o n a l s o l u t e . U s e of higher o r d e r a p p r o x i m a t i o n s of s o l u t i o n t h e o r y 6'7 does not change this conclusion. Thus, simple pairwise bond-energy m o d e l s of s o l u t i o n s c o n t a i n i n g an i n t e r s t i t i a l solute p r e dict that logeT~ in a m u l t i c o m p o n e n t solution should be the c o m p o s i t i o n - w e i g h t e d m e a n of its v a l u e s in the p u r e c o m p o n e n t s . Since this is c o n t r a r y to much e x p e r i m e n t a l e v i d e n c e , m o d e l s which do not involve the c o n s t a n t p a i r w i s e b o n d - e n e r g y a s s u m p t i o n have been proposed. Belton and T a n k i n s 8 and J a c o b an
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